In the production of crude oil, it is sometimes found that the crude oil is sufficiently viscous to require steam injection into the petroleum reservoir. Ideally, the petroleum reservoir would be completely homogeneous and steam would enter all portions of the reservoir evenly. However, it is often found that this does not occur. Instead, steam selectively enters a small portion of the reservoir while effectively bypassing other portions of the reservoir. Eventually, "steam breakthrough" occurs and most of the steam flows directly from an injection well to a production well, bypassing a large part of the petroleum reservoir and producing significant amounts of wasted heat.
It is possible to overcome this problem with various remedial measures, e.g., by using foam or by plugging off certain portions of the injection well. For example, see U.S. Pat. Nos. 4,470,462 and 4,501,329 which are hereby incorporated by reference herein. However, to institute these remedial measures, it is necessary to determine which portions of the reservoir are selectively receiving the injected steam. This is often a difficult problem.
Various methods have been proposed for determining how injected steam is being distributed in the wellbore. D. E. Bookout et al. in a publication entitled "Injection Profiles During Steam Injection", SPE Paper No. 801-843 C, pp. 1-16, dated May 3, 1967 summarizes some of the known methods for determining steam projection profiles.
The first and most widely used of these methods is known as a "spinner survey". In this method, a tool containing a freely rotating impeller is placed in the wellbore. As steam passes the impeller, it rotates at a rate which depends on the velocity of the steam. The rotation of the impeller is translated into an electrical signal which is transmitted up the logging cable to the surface where it is recorded on a strip chart or other recording device.
As is known to those skilled in the art, these spinners are greatly affected by both the quality and the rate of steam injection into the well. Steam quality and rate decrease with depth leading to unreliable and unreproducible results.
Radioactive tracer surveys are also used in many situations. With this method methyl iodide (131) has been used to trace the vapor phase. Sodium iodide has been used to trace the liquid phase. Radioactive iodine is injected into the steam between the steam generator and the injection well. Injected tracer moves down the tubing with the steam until it reaches the formation, where the tracer is temporarily held on the face of the formation for several minutes. A typical gamma ray log is then run immediately following the tracer injection. The recorded gamma ray intensity at any point in the well is then assumed to be proportional to the amount of steam injected at that point.
Vapor phase tracers have variously been described as alkyl halides (methyl iodide, methyl bromide, and ethyl bromide) or elemental iodine. The arrival of the radioactive tracer is a function of steam quality. Since quality decreases with depth then most of the material would be dissolved in the liquid phase in the lower portion of the injection well leading to unreliable measurements.
Although these methods can be utilized, what is needed is a simple and efficient method for determining the rate of steam injection and the quality of the steam injected. It is important to determine which intervals of the formation are accepting steam and the quality of the steam both at the injection wellbore and at an observation well away from the injector.